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Example of Collections.singleton in Java

By Arvind Rai, May 31, 2013
On this page we will provide the use of java Collections.singleton, Collections.singletonList and Collections.singletonMap. They will return serialized and immutable java Set, List and Map respectively containing the given object. Find the example.

Collections.singleton

It returns a serialized and immutable Set containing given object.
CollectionsSingleton.java
package com.concretepage.util;
import java.util.Collections;
import java.util.Set;
public class CollectionsSingleton {
     public static void main(String[] args) {
         User user1 = new User("Mahesh");
         Set<User> set= Collections.singleton(user1);

         User user2 = new User("Ramesh");
         //will throw error
         set.add(user2);
     }
 }
User.java
package com.concretepage.util;
public class User {
   private String name;
   public User(String name) {
       this.name = name;
   }
   public String getName() {
       return name;
   }
}
Output
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.util.AbstractCollection.add(AbstractCollection.java:262)
	at com.concretepage.util.CollectionsSingleton.main(CollectionsSingleton.java:11) 

Collections.singletonList

It returns a serialized and immutable List containing given object.
CollectionsSingletonList.java
package com.concretepage.util;
import java.util.Collections;
import java.util.List;
public class CollectionsSingletonList {
     public static void main(String[] args) {
         User user1 = new User("Mahesh");
         List<User> list= Collections.singletonList(user1);
         System.out.println(list.get(0).getName());

         User user2 = new User("Ramesh");
         //will throw error
         list.add(user2);
     }
 }

Collections.singletonMap

It returns a serialized and immutable Map containing given key and value.
CollectionsSingletonMap.java
package com.concretepage.util;
import java.util.Collections;
import java.util.Map;
public class CollectionsSingletonMap {
     public static void main(String[] args) {
         User user1 = new User("Mahesh");
         Map<Integer, User> map= Collections.singletonMap(1, user1);
         System.out.println(map.get(1).getName());

         User user2 = new User("Ramesh");
         //will throw error         
         map.put(2, user2);
     }
 }
POSTED BY
ARVIND RAI
ARVIND RAI
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